Mammalian colon is responsible for the final stages of intestinal water and electrolyte absorption. Intestinal absorption is brought about by active transport mechanism found in membranes of the epithelial cells lining the lumen of the intestine. Work proposed here will examine the cellular mechanisms of C1 absorption and secretion by rabbit colon. This tissue actively absorbs Na and C1 and secretes HC03. K ion is passively secreted by the tissue. Rabbit colon will also actively secrete C1 under certain pathophysiologic conditions. In view of the suggestion that a C1:HC03 exchange mechanism is responsible for active C1 absorption this proposed research has the following specific aims: (i) to confirm the existence of a C1:HCO3 exchange in the mucosal cell membrane; (ii) to determine the driving forces for C1 and HCO3 movement across both limiting cell membranes; and (iii) to examine the role of metabolism in this transport phenomenon. Since a Na-coupled mechanism for active C1 secretion has been suggested, this work will have the following additional specific aims in tissues stimulated to secrete C1: (iv) to determine if C1 entry at the basolateral cell membrane is driven by the entry of Na; and (v) to identify the epithelial cell types or regions responsible for active C1 secretion. These studies, in isolated tissues, will use conventional ion tracer flux and electrophysiologic methods to determine active and passive ion transport rates across this epithelium. In addition, conventional and ion-selective microelectrodes (C1 and pH) will be used to determine driving forces for ion transport across the limiting cell membranes. Experimental manipulations of the tissues that will be used to reveal transport mechanisms include studies: (i) in the control state; (ii) in the short-circuited state; (iii) in the absence of Na transport (blocked by amiloride or Na-free bathing solutions); (iv) in the presence of C1 transport inhibitors (DIDS; piretanide); (v) with carbonic anhydrase (acetazolamide) and metabolic (IAA) inhibitors; and (vi) with agents that stimulate active C1 secretion (c-AMP; A23187).